With rapid development of economy and urbanization, energy crisis, environment pollution, and traffic congestion are becoming more and more serious in the worldwide. City electric transportation vehicles (such as metro, tram, and trolley bus) and electric vehicles may relieve increasingly serious urban traffic congestion problem, reduce consumption of limited fossil fuel, reduce air pollution, realize energy-saving and emission-reducing and sustainable development of city, thus becoming a major vehicle to be developed in future city.
Development of electric vehicle industry requires corresponding charging system. There are normally three kinds of charging modes: normal charging, rapid charging, and power batteries rapid swapping system (briefly referred as to battery-swapping). Whereas battery-swapping is a better charging mode for urban public transportation vehicles. Electric vehicle charging system needs not only huge construction cost, but also will have bad effects on power system. Furthermore, power batteries of electric vehicle are also an energy storage equipment of huge capacity.
A conventional DC traction power supply system generally utilizes diode rectifiers to realize power supplying. There are two kinds of manners by which a traction substation provides power to a catenary: unilateral power supply and bilateral power supply. And DC power supply voltage ratings as set forth in China Standard are 750V and 1500V. Braking energy of an electric locomotive often is lost in braking resistors. This not only causes energy consumption, but also causes rising of temperature in tunnel, thereby burdening temperature control system and causing further energy consumption. Recycling of electric locomotive braking energy is an object to be realized in future in traction power supply system.
As shown in FIG. 1, a prior art DC traction power supply system includes one or more traction substations 1 for offering DC energy to an electric locomotive 2. The traction substation 1 runs based on a conventional diode rectification DC traction power supply system. Each traction substation 1 is composed of a plurality of transformers 11, a plurality of rectifiers 12, a DC bus 13, a catenary 14, a steel rail 15 and a section post 16. Wherein, each transformer 11 may be a two-winding transformer, three-winding transformer, or another kind of three-winding transformer of which the primary winding is connected in a prolonged delta manner and phase shift is ±7.5°. Each rectifier 12 may be of a six-pulse, twelve-pulse or twenty four-pulse rectifier.
For a conventional DC traction power supply system, during operation valley and nonoperation period of the electric locomotive, the main substation at AC end has low power factor and therefore requires reactive power compensation measurement.
Fast progress of economy promotes quick growth of electric power demand. Conventional centralized large centralized power grid suffers from high cost, great difficulty in operation, and is hard to meet higher and higher safety and reliability requirement of the user. Compared to centralized power generation, a distributed system has the advantages of less pollution, high energy usage, and is flexible to be mounted in suitable site. As such, it reduces operation cost and resources of transmission and distribution, reduces line loss of centralized power supplying, reduces total capacity of the grid, improves grid peak and valley performance and reliability and accordingly, it is strong assistance and effective support for large centralized power grid, and represents one of development trends of the power system.
The distributed generation includes solar photovoltaic cell, a wind turbine generator, a micro gas turbine, a fuel cell, and biomass power generation and the like.
A single distributed generation has high access cost, and difficulty in control. Compared to a large centralized power grid, it is uncontrolled power source and as a result, this distributed generation is often handled by the large centralized power grid by means such as limitation and isolation with the purposes of decreasing its impact on the large centralized power grid, hence significantly restricting usage of distributed generation.
Sufficiently exploiting urban electric public transportation, distributed generation such as solar photovoltaic cell, micro gas turbine and fuel cell may be realized.
A microgrid is an organic system constructed collectively of load, micro sources (such as distributed generation in the microgrid, for example photovoltaic generation, wind generation or the like), energy storage devices and electric power transformation devices. For power supply inside the microgrid, power electronic devices mostly perform energy transformation, and provide necessary control. The microgrid is able to effectively overcome problems of access the above distributed generation to the large centralized power grid, and coordinate conflict between the large centralized power grid and distributed generation.
A microgrid may be classified into AC microgrid, DC microgrid, high frequency microgrid and AC/DC hybrid microgrid. As the solar photovoltaic cell and fuel cell of the distributed generation are of DC type, while the wind turbine generator, micro gas turbine and biomass power generation are of AC with different frequency which can be converted into DC current through one time rectification. DC microgrid has no skin effect, lower loss, higher efficiency, and has no need for reactive power compensation. It has no stability problem existing in AC power supply system, and its transmission distance and power is not restricted by power system inherent operation stability, and it is easy to control. Therefore, it has extensive development and application prospect.